by John McPhee
In the early Pliocene, a volcano grew into the range there. It has long since eroded away. Andesite lavas poured from the volcano. Lighter eruptive material settled around the crater. In the moist atmosphere, the volcano’s eruptions caused prolonged heavy rains. The water mobilized the unstable slopes. Volcanic muds—full of the sharp rock fragments that would cement together as breccia—slid into the country. In quiet periods between eruptions, streams flowing down the volcano tumbled some of the rock fragments, rounding pebbles. In recent time, alpine glaciers dug into the country and dozed away much of what was left of the volcano, and as the ice melted it left upon the brecciated mudflows heaps of lateral till. (“It is mostly andesite mudflow breccia with reworked stream gravel in it and glacial till on top, which appears to be moraine but is not.”)
All this had happened in one areal spot. All this was represented in that one roadcut. Anyone could be pardoned if, at first glance, the complete narrative seemed less than apparent. The story had repeated itself through much of the Sierra during the same band of time: other volcanoes extruding andesite and shedding mud, their remains disturbed by ice. It was a surface story, a latter-day account. The brecciated mudflows and andesite lava flows had come to rest on rock that was older by as much as five hundred million years—rock with a deep and different story, rock that just happened to be there when the mountains rose. The layers of the Grand Canyon are full of the temporal stratigraphic gaps known as unconformities. In the Grand Canyon, much more time is absent than is represented. If a gap of five hundred million years were the right five hundred million years, it could erase the Grand Canyon. In eastern California, the infinitesimal space between the andesite flows and the rock on which they hardened is known as the Great Sierra Nevada Unconformity. To understand what that was and how it had come to be was to understand the relationship between just two of the parts in a millipartite structure.
Moores and I went on to California’s eastern boundary, turned around, and recrossed the Sierra, as we would do repeatedly in the coming years. Climbing the steep east face of the mountains, you see granite and more granite and andesite capping the granite. So far so comprehensible. But before you have crossed the range you have seen rock of such varied type, age, and provenance that time itself becomes nervous—Pliocene, Miocene, Eocene non-marine, Jurassic here, Triassic there, Ypresian, Lutetian, Tithonian, Rhaetian, Messinian, Maastrichtian, Valanginian, Kimmeridgian, upper Paleozoic. The rocks seem to change as fast as the traffic. You see olivine-rich, badly deformed metamorphic rock. You see serpentine. Gabbro. One thing follows another in a manner that seems random—a collection of relics from varied ages and many ancestral landscapes, transported from far or near, set beside or upon one another, lifted en masse in fresh young mountains and exposed in roadcuts by the state. You cannot be expected, just by looking at it, to fit it all together in mobile space and sequential time, to see in the congestion within this lithic barn—this Sierra Nevada, this atticful of objects from around the Pacific world—the events and the vistas that each item represents.
Suppose you were to find in a spacious loft a whale-oil lamp of pressed lead glass. What would you think, know, guess, and wonder about the origin and the travels of that lamp? And suppose you were to find near it a Joseph Meeks laminated-rosewood chair, and an English silver porringer and stand, and an eight-lobed dish with birds in a flowering thicket. It is possible that you would not immediately think 1850, 1833, 1662, and 1620. It is possible that you would not envision the place in which each object was made or the milieu in which it was first used, and even more possible that you would not discern how or when any of these pieces moved through the world and came to be in this loft. You also see, lined up in close ranks, a Queen Anne maple side chair, a Federal mahogany shieldback side chair, a Chippendale shell-carved walnut side chair, and a William and Mary carved and caned American armchair. Stratigraphically, they are out of order. How did that happen? Why are they here? Only one thing is indisputable: this is some loft. Jammed to the trusses, it also contains a Queen Anne carved-mahogany blockfront kneehole dressing table, a Hepplewhite mahogany-and-satin-wood breakfront bookcase, a rosewood Neo-Gothic chair, an Empire mahogany step-back cupboard, and a Regency mahogany metamorphic library bergere. It contains a classical brass-mounted mahogany gilt wood-and-gesso bed with pressed-brass repousse. It contains a Federal cherry-wood-and-bird’s-eye-maple bowfront chest of drawers, an early Victorian mahogany dining chair with a compressed balloon back, a Federal carved and inlaid curly-maple-and-walnut fall-front desk, a Windsor sackback writing armchair, and a Louis XV ormolu-mounted kingwood parquetry commode. There’s a temple bell dating to Auspicion Day of the fifth month of the first year of Tembrun. There’s a Federal carved-mahogany armchair with a cornucopian splat.
Sort that out. Complete a title search for each piece. Tell each story backward through shifting space to differing points in time. Imagine the palace, the pavilion, the house, the hall for which each piece was fashioned, the climate and location of the country outside.
Naturally, you can’t do that—not in a single reconnaissance. Don’t fret it. Don’t fret that you can’t see the story whole. You cannot tell whence each of these items has come, any more than its maker could have known where it would go.
“Nature is messy,” Moores remarked. “Don’t expect it to be uniform and consistent.”
I remembered the sedimentologist Karen Kleinspehn saying to me in these same mountains, “You can’t cope with this in an organized way, because the rocks aren’t organized.”
Gradually, though—outcrop to outcrop, roadcut to roadcut—Moores revived enough related scenes in the distinct origins of the random rock to frame a cohesive chronological story. That is what geologists do. “You spend a lot of time working over rocks and you have a lot of time to do nothing but think,” he said. “These mountains, for example, are Tertiary normal faulted, confusing the topography with regard to structure. They show different levels of structure in different places. To see through the topography and see how the rocks lie in three dimensions beneath the topography is the hardest thing to get across to a student.” After a mile of silence, he added cryptically, “Left-handed people do it better.”
I said nothing for a while, and then asked him, “Are you left-handed?”
He said, “I’m ambidextrous.”
As it happens, I am left-handed, but I kept it to myself.
From the east, the climb is rapid to Donner Summit—less than thirty miles, and the road is not straight. Yet elsewhere along the Sierra front the rise is so much shorter and steeper that nothing on wheels could ever climb it. From the basin below (altitude four thousand feet), you bend your neck and look ten thousand feet up a granite mass that was lifted intact, whereas here, on the route above Reno, the “Tertiary normal faulting” that Moores referred to has tiered the escarpment and lowered the crestline as well. The early trappers found a native trail here. In all likelihood, the natives who made the trail were animals, followed, in time, by people.
Under ponderosas and western cedars at the Nevada-California line, the granite reveals itself and then is quickly gone, as the roadside rock becomes something like dark cordwood, fallen in columnar blocks. This is the caprock andesite, which cracks into columns as it cools. Another five miles, and the interstate moves through a long cut that is buff, gray, buff, gray, and buff again as lava flows and mudflows intersperse. Perhaps a hundred thousand years separate the lava flows, while the laminating muds come ten times as often. The volcanic cap over the granite is still a kilometre thick here. Among the trees are erratic boulders—granite boulders out of place on the andesite, transported a few thousand years ago by a descending ribbon of ice.
Three miles before the summit, the granite reappears, not in ice-transported bits but in bedrock at the side of the road. And then more granite, under Jeffrey pines—weathered granite, light and sparkling sliced granite. It ends abruptly, at a contact with andesite. This particular granite had bee
n sitting here eroding quietly for maybe ninety million years when the andesite lava flowed upon it, coating hills and filling valleys, plastering over the granitic terrain, concealing and preserving a Miocene landscape. Differentially, randomly, erosion has eaten through the caprock. So the road encounters both formations. Granite reappears at the summit.
Donner Summit, at seven thousand two hundred and thirty-nine feet, is half the height of the range. Locally, engineers found a way for the interstate which is considerably less precipitous than the trail used by the emigrants in the eighteen-forties. The place that came to be known as Donner Pass is a couple of miles south, on a relic stretch of U.S. 40. Moores and I once went over there and stood on a cliff edge, looking east. Tens of thousands of square miles of basin-and-range topography fanned out into Nevada, all of it aimed, within converging lines, at the pass. The drop to Donner Lake, more than a thousand feet below, was almost giddy. To get over the pass, everything on feet or wheels had to come up that grade. In a normal year, about seventy inches of water falls on the High Sierra, nearly all of it as snow. Seventy inches of water is roughly one and a half times what falls on New York City and twice what falls on Seattle. The snow on the Sierra Nevada can be forty feet deep. At the end of October, 1846, the Donner party came up to this pass and were forced to retreat by a mountain of snow. The winter camp where they starved and died was by the shore of Donner Lake, in the cirque below the pass.
In deep winter, I have stayed near Donner Lake in a ski condo where a previous guest left a peevish note: “The peace and beauty are marred by a noisy refrigerator and heating unit.” Now, in midsummer, there were, around the pass, spreads of tenacious snow. A bicyclist, standing as he pumped but scarcely puffing, came up the route of the emigrants. Seating himself as he reached the zenith, he coasted on to the west. To the east, the deep gulf of scenery that he had come out of owed itself less to the finishing touches of ice than to large parallel north-south faults that had lowered a large piece of country—a crustal block, dropped between two other crustal blocks, and now a graben. Lake Tahoe, southeast across a partitioning ridge from Donner Lake, lies in the same graben. The small lake and the large one would be connected but for a recent pouring of andesite, which formed the ridge.
Moores said to notice how the mechanical lowering of a large piece of the mountains had caused varying levels of the original structure to turn up in unexpected places. To try to sense a structure, he repeated, one must develop a talent for “seeing through the topography” and into the rock on which the topography was carved. When rocks in their variety arrive in a given place, like furniture going into storage, they hold within themselves their individual histories: their dates of solidification, their environments of deposition, or their metamorphic experience, as the case may be. Their unit-tounit relationship—their stratigraphy and other juxtapositions—pondered as a whole is structure. Structure on the move is tectonics.
When topography is as beautiful as at Donner Pass, it is not an easy matter to see through it, but if you’re looking for structure you might start with the granite. In all the country from Nevada to the pass, the volcanic cap makes its appearances, but always as veneer —eroding everywhere, opening windows, and ultimately suggesting the bewildering mass of the underlying granite. This is the Sierra batholith. Geologists reserve that term for the largest bodies of magmatic rock. A batholith, as defined in the science, has a surface of at least forty square miles and no known bottom. For the latter reason, it is also called an abyssolith. The one in California has a surface of about twenty-five thousand square miles. It lies inside the Sierra like a big zeppelin. Geologists in their field boots mapping outcrops may not have been able to find a bottom, but geophysicists can, or think they can, and they say it is six miles down. If so, the batholith weighs a quadrillion tons, and its volume is at least a hundred and fifty thousand cubic miles.
It reminds me of a big rigid airship because the rigids contained, within their metal frames, rows of giant bags that resembled aerial balloons. Batholiths develop not as single chambers of magma but as contiguous balloons of molten rock called plutons. As red-hot rising fluid, the great Sierra batholith came into the country in successive pulses during a hundred and thirty million years between early Jurassic and latest Cretaceous time. There were three peak periods—the first nearly two hundred million years before the present, the second at a hundred and forty million years before the present, and the third at eighty. The most extensive is the “80 pulse.” All this went on some ten to thirty kilometres below the earth’s surface, where continental crust and subducting ocean crust (coming under the continent) were melting. Through Maastrichtian time and nearly all the Cenozoic epochs, the cooled and cooling magma lay buried. The topography above changed and changed again, like a carrousel of slides. And eventually, recently, the batholith came up, to serve as the lithic medium for the erosive sculpting of Olancha Peak, of Wheeler Peak, of Mt. Whitney.
Dark cliffs above Donner Pass were Pliocene volcanics, but the rock beside the trail was granite—poetically weathered organic billows of granite. There were small black shapes within it. Thousands of them. Alien pebbles. These were bits of the country rock that the batholith intruded. They had fallen into the magma while it was still molten or, if cooler than that, sufficiently yielding to be receptive. They had been softened and rounded but not melted and destroyed. On Interstate 80 west of Donner Summit, we saw larger chips of such metasediment in the granite of the roadcuts. Another mile, and they were larger still. Moores referred to them as “abundant xenoliths—Jura-Triassic pieces of the wall or the roof.” These were not the andesites and other outpourings that had been spread upon the granite in fairly recent times; these were parts of the intruding batholith’s containing walls or roof. They had fallen into the soft granite eighty million years ago, and, before that, had been crustal rock for something like a hundred million years.
In the interstate median, under Jeffrey pines, were bedrock outcrops that had been scoured and polished by overriding ice eleven thousand years ago. There were plenty of erratic boulders. For eleven miles after Donner Summit, the xenoliths in the granite increased in volume until what we had first seen as pebbles were now the size of bears. To sense the implication of what was coming, a structural geologist would need no further sign. We were fast approaching the wall of the batholith—the magma’s contact with the country rock. That highway engineers would blast out a roadcut at just such a place is fortuitous, a matter of random chance, but when Ken Deffeyes and I had come into this same right-hand bend he had shouted, “Whoa! Whoa! Pull over!” And a moment later he was saying, “This is the best outcrop on all of I-80. You can walk up and touch the wall of the great batholith.”
Moores now called it “about as classic and neat a contact as you’ll ever see.” As cars shot past us like F-18s, he added, “Right here. Bang!” The contact was essentially vertical. It ran on up the mountainside and vanished under the trees. It could not have been more distinct had it been the line between a granite building and a brick building adjacent in a city. The granite of the batholith looked almost white beside the reddish country rock, which Moores described as the metamorphosed remains of what had once been an island arc. The granite was customary, competent—a lot of salt and less pepper. The arc rock was flaky, slaty—like aged iron in a state of ulcerated rust. In the first yards after the contact, tongues of granite reached into the country rock, preserved in the act of eating xenoliths. Within a short distance, they gave up.
As the rock ran on in the long continuous cut, it turned black, burgundy, buff, and green, in vertical stripes, in tight drapefolds with long limbs. Obviously, it had been caught up—before the arrival of the batholith—not in some minor local slumpage but in a regional and pervasive tectonic event. With two more miles, the story again made a radical change, as we came to a roadcut of gabbro. Charcoal-gray and sparkling, it was perfect gabbro. There are rich, handsome houses on the Upper East Side of Manhattan that are made of
less perfect gabbro. Gabbro, too, is cooled magma. Lacking quartz, it is at the dark end of a spectrum the light end of which is, for the most part, granite. Peridotite—the rock of the earth’s mantle—was in the roadcut as well, and Moores said that in his opinion these mafics and ultramafics (rocks low in silica and high in magnesium and iron) arrived after the event that had drapefolded the rock up the road and before the intrusion of the batholith. As we moved on, the gabbro-peridotite interdigitated with granite and then disappeared as the road once again descended into the Sierra batholith. After corridors of granite, there were more volcanics, in the topographic scramble of structure.
When panoramic views came along, they showed the uniformity of the sixty-mile slope—the low-angle plane of the western Sierra. The great surface (the top of the trapdoor) was completed in the eye rather than the rock. It was deeply eaten out by river gorges. To the north and the south, the vistas were wide over deep valleys to tilting planar skylines. We came to Emigrant Gap, where the erosional dissection was particularly deep. Nineteen miles from Donner Pass, the scene demonstrated with emphasis that once emigrants were across the summit they were scarcely free of trouble. From Emigrant Gap into Bear Valley they lowered their wagons on ropes. We looked into the valley, where an alpine meadow was flanked with incense cedars. Above it to the north, under the smoothly sloping skyline, were west-dipping sediments that Moores described as mudflow breccias over Paleozoic sandstones. A deep gorge cut through this ridge. It contained the Yuba River, where the Yuba, with the help of alpine ice, had captured the Bear. To the northeast, under high white peaks, was a lake gouged in granite by an alpine glacier, which had left its moraines on the volcanic muds among the sharp shards and round pebbles that had caused Deffeyes to throw in his towel. Rocks between us and that lake, Moores said, were “lower Paleozoic quartz-rich sediments metamorphosed and folded at least twice.” And the rocks in the peaks above the lake were remains of a Jurassic island arc.